1. Field of the Invention
The present invention relates to a roller bearing for an infinite rectilinear motion and, more particularly, to a roller bearing of this type including a long truck rail and a slide unit which is arranged so as to ride over the truck rail and relatively performs the infinite rectilinear motion along the truck rail using cylindrical rollers as rolling elements.
Practically speaking, this invention relates to a roller bearing for an infinite rectilinear motion of what is called a parallel roller type in which cylindrical rollers are arranged on the truck surface of a truck rail where these rollers roll in a manner such that the central axes of the rollers are parallel with each other and are directed in the same direction.
2. Prior Arts
The roller bearings for an infinite rectilinear motion are classified into the cross roller type and the parallel roller type in dependence on a method of arrangement of the rollers. In the cross roller type, the central axes of the rollers perpendicularly cross each other in the rectilinear passage of the infinite circulating passage. In the parallel roller type, the respective rollers are arranged such that their central axes are directed in the same direction in the rectilinear passage.
In the case of the cross roller type, a single flow of rolling rollers is respectively provided on both of the right and left sides of the truck rail, namely, total two flows of rolling rollers are provided, thereby making it possible to load a weight in every direction. Thus, the whole bearing size is miniaturized and the number of parts is also reduced.
However, in the case of the cross roller type, a weight in a fixed direction is loaded by every other rollers in the truck surface. Therefore, to increase the rated weight, the load performance of the bearing must be increased by making the length of truck groove long. Therefore, if the cross roller type is used, the bearing becomes long in the slide direction and miniaturization of the bearing is obstructed.
On the other hand, in the case of the bearing of the parallel roller type in which the rollers in the rectilinear passage are arranged such that their central axes are directed in the same direction, all of the rollers in each truck groove can share the weight, so that this type of bearing has a high weight loading performance. However, in order to load the weight in every direction, every two flows of rolling rollers, namely, total four flows of rolling rollers are necessary for the right and left sides. An increase in number of flows of rolling rollers causes the height of cross section of the bearing to be enlarged.
The case of the parallel roller type bearing will be explained with reference to FIG. 1. In this case, since FIG. 1 is symmetrical in the horizontal direction with respect to a center line C, reference numerals are written on only the left side of the center line C. When a slide unit 1 is mounted so as to ride over a truck rail 2, rollers 3 and 3A are arranged at a predetermined angle of inclination (45.degree.) for the center line C in a manner such that the extension lines (not shown) of the central axes (rotational axes) of the rollers 3 and 3A cross perpendicularly each other in order to also load the weight in every direction. Return passages 4 and 4A are formed in the slide unit 1. Two truck grooves in which the rollers 3 and 3A exist are communicated with the return passages 4 and 4A through direction change passages 5 and 5A.
In order to enable the rollers to smoothly roll in the truck grooves, direction change passages 5 and 5A, and return passages 4 and 4A, the return passages 4 and 4A need to be formed at the positions which are inclined with regard to the horizontal directions from two truck grooves and at the same time, the angles of inclination for the horizontal lines of the return passages 4 and 4A need to be the same as the angles of inclination (45.degree.) of the rollers 3 and 3A for the center line C. A height h.sub.1 of the slide unit 1 which satisfies the above conditions is, in general, very larger as compared with the case of the cross roller type bearing. Namely, according to the conventional parallel roller type bearing, the height of cross section of the bearing cannot help increasing in terms of the motion characteristics of rollers (for example, refer to Japanese Patent Application No. 181431-1982, and the like), and the number of parts also increases and the bearing becomes expensive.
To reduce the height of slide unit, there has been proposed a method whereby the truck grooves, direction change passages 5 and 5A, and return passages 4 and 4A are arranged in a positional relation as shown in FIG. 2. In this case, a height h.sub.2 of the slide unit 1 is fairly smaller than the height h.sub.1 in the case of FIG. 1. However, it is remarkably difficult to work and form the inside of the slide unit 1 such that the direction change passages 5 and 5A don't mutually cross, and very complicated and difficult machining processes are necessary. The manufacturing cost is also high.